Conventionally, with the aim of controlling access to image data and the like, the image data is encrypted and scrambled. More specifically, the image data is encrypted using an encryption key, whereby only the person who has the decryption key corresponding to the relevant encryption key can correctly reproduce the encrypted image data.
Partial encryption is known as one of the methods of encrypting the image data. More specifically, partial encryption is a technique of not encrypting data of the whole image but instead selecting and encrypting a partial area of the image data (for example, see U.S. Pat. No. 6,473,859).
Further, partial decryption is known as one of the methods of decrypting the encrypted image data. More specifically, partial decryption is a technique of not decrypting encrypted data for a whole image but instead selecting and decrypting a partial area of the encrypted image data (for example, see Published United States Patent Application 2003/0190042).
Moreover, multiple encryption, which involves encrypting the image data several times, is known. Here, multiple encryptions is a technique to be used when the encrypted image data is further encrypted.
That is, by appropriately using partial encryption, partial decryption or multiple encryption, it is possible to achieve flexible control of access to the image data.
However, in the past, partial encryption, partial decryption and multiple encryption have not been used in combination, and a method of reproducing an image in a case where partial encryption, partial decryption and multiple encryption are appropriately combined and applied to image data has not been disclosed.
Incidentally, in a case where image data subjected to multiple encryptions in a certain encryption order is decrypted, the decryption must be executed in the order opposite to the certain encryption order. In that case, the following problems may occur. That is, if the partial-encrypted and multiple-encrypted image data is subjected to partial decryption, it is impossible to specify which of the relative areas has been subjected to the multiple encryptions. Thus, there is a possibility that a key which is unnecessary for the decryption is unnecessarily acquired. In addition, it is impossible to specify the decryption order with respect to the areas to be decrypted and reproduced. Thus, there is a possibility that the partial decryption cannot be executed, whereby the whole areas subjected to the multiple decryptions are unnecessarily decrypted.